Conventional data communication systems have been applied to accomplish object identification using the medium of radio broadcast. Such radio frequency identification (RFID) systems find application in the fields of materials handling, inventory control, and generally in the field of tracking personnel, objects, and animals. Identification devices are of three known types: passive, active identification devices of the beacon type, and active identification devices of the transceiver type. In an exemplary arrangement, a system may include an interrogator (or system receiver for beacons) and several thousand identification devices. Each identification device is typically packaged as a low-cost or disposable label or tag and placed on an object, animal, or person to be tracked. Each identification device is manufactured using integrated circuit technology, programmed with a unique identifier, and assembled with a printed circuit antenna to form a flat assembly for incorporation into the label or tag. Typically, the interrogator (or system receiver) has a fixed location, while identification devices move from time to time in and out of the communication field of the interrogator (or system receiver). It is highly desirable to accurately and quickly identify identification devices from a population which may number in the billions. At the same time, it is highly desirable to reduce the cost of each identification device.
Accurate and reliable detection of identification devices is made difficult by a number of factors including, for example, (a) identification devices have a limited amount of power available to operate when required to provide a radio transmission; (b) the orientation of the identification device antenna may be unsuitable for absorbing sufficient power from a signal transmitted to it; (c) the orientation of the antenna of the identification device may be unsuitable for providing a transmitted signal sufficient for accurate reception by the interrogator (or system receiver); (d) multiple signals may arrive at an interrogator (or system receiver) based on the same transmission by an identification device due, inter alia, to radio frequency reflections in the environment; and (e) identification devices that transmit simultaneously may cause a so-called collision.
There remains a need for an object identification system suited for coordinating the use of a common medium among potentially billions of identification devices for detection, interrogation, and/or control activities to be accomplished in a limited time. In addition, there remains a need in some applications to minimize the circuitry, firmware, and software complexity required at each identification device, to extend the operating range of communication, and to support larger numbers of individual identification numbers perhaps at the expense of complexity at the interrogator (or system receiver). Without these improvements, the size and cost per identification device cannot be reduced to permit new and improved object identification systems that employ inexpensive disposable identification devices such as identification tags, baggage tags, inventory labels, and the like.